1. Field of the Invention
The invention relates generally to disk drives. More specifically, the invention relates to a system for controlling a spindle motor in a switched or chopped voltage mode.
2. Description of the Prior Art
Disk drive manufactures are continually striving to increase sustained data transfer rates of their disk drives. Sustained data transfer rates can be increased by increasing the spin rate of the disk drive's platters, which store encoded data. The faster the platters can be spun, the faster data can be written to, and read from, the platters. The faster spin rates, in turn, can be realized by utilizing bigger, faster spin motors, having larger windings and higher currents flowing through the windings.
Increasing the size and speed of the spindle motor places a greater demand on the system for controlling the speed of the motor. A current control maintains the spin rate of the motor at a near-constant level. For example, a spindle motor having a design speed of 5400 rpm is controlled to spin within 5.5 rpm (.+-.0.1%) of that design speed. If the current control allows the motor to spin more than 5.5 rpm above the design speed, data might be mistakenly written to a servo burst area on the track of a platter. Consequently, all data on that track would be lost, and the track itself would become permanently unusable.
Linear current controls are not well-suited for controlling the bigger, faster spin motors. As speed and power increase, driver power dissipation becomes excessive. Therefore, attention has been focused on switched driver techniques.
Switched current controls using current feedback have been used in disk drives. The switched driver techniques reduce the problem of driver power dissipation. However, the switched current controls have inherent stability problems. Overcoming the stability problems requires the addition of complex circuitry, which adds to the cost of the disk drive. Additionally, the switched current controls have acoustic problems.
Switched voltage controls without voltage feedback have been considered impractical for disk drives. Variations can occur in the power supplied to the spindle motor. The variations can be caused by CD ROM drives and other devices sharing power with the disk drive. Without voltage feedback, the voltage control does not stop a voltage spike from increasing the speed of the spindle motor and causing data to be mistakenly written to a servo burst area on the track of a platter.
Consequently, disk drive manufacturers have chosen to stay with expensive and inefficient current controls for regulating motor current. In the very cost-competitive, cost-sensitive market for hard disk drives of personal computers, however, the additional expense of the current control is borne almost entirely by the disk drive manufacturer.
An inexpensive and efficient system for controlling the speed of the spindle motor is needed.